1. Field of the Invention
The present invention relates to an alternator (an AC-generator) to be mounted on an automotive vehicle.
2. Description of Related Art
An alternator having a structure for preventing foreign particles from entering into a bearing portion has been known hitherto. An example of a conventional structure for preventing foreign particles is shown in FIG. 5 attached hereto. In the structure shown in FIG. 5, a cooling fan 110 connected to a rear side of a rotor includes an annular portion 112 bent from the cooling fan in the axial direction. A rear side bearing 114 supporting the rotor is held by a bearing holder 118 that extends in the axial direction. The annular portion 112 overlaps with the bearing holder 118 in the axial direction while making a small gap therebetween in the radial direction, so that a labyrinth is made between the annular portion 112 and the bearing holder 118. Thus, foreign particles contained in cooling air are prevented from entering into the bearing 114.
Another example of the structure for preventing foreign particles is disclosed in JP-A-2005-151728. A relevant portion of the structure is shown in FIG. 6 attached hereto. An inner ring of a bearing 120 rotates together with a rotor shaft 130. A pair of shield plates 122, 124, each having an L-shaped cross-section, are connected to the rotor shaft 130 so that they rotate together with the inner ring of the bearing 120. Foreign particles entered to the vicinity of the bearing 120 are blown away in the radial direction by a centrifugal force of the shield plates 122, 124.
In the conventional structure shown in FIG. 5, a length of the annular portion 112 in the axial direction has to be made longer to further improve its ability for preventing foreign particles from entering the bearing portion. However, a foot portion 119 becomes thinner by making the axial length of the annular portion 112 longer. A mechanical strength of the foot portion 119 may not be strong enough to endure repetition of changes in rotational speed of the rotor. In the conventional structure shown in FIG. 6, the shield plates have to be additionally prepared and connected to the rotor shaft 130. This makes a manufacturing cost of the alternator higher.